There generally are two systems for applying driving force to a plurality of rolling mills arranged along a rolling line for rolling continuous length sections such as bar, wire rod or the like such that the rolling mills may be provided with respective motors to be driven separately, or that a plurality of rolling mills are interlocked through a driving mechanism to be given a driving force by a single motor (this is herein called the "common drive system"). In the former system providing the respective motors for each rolling mill, the rotating speed of the motors are set separately in consideration of variance of tension applied to the rolled material between the rolling mills corresponding to variance of area reduction ratio of the material being delivered from each of the stands in question. By contrast, the common drive system of the prior art lacks versatility in that, once a specific gear ratio is set in a gear transmission assembly, the relative roll speed of the two stands cannot be varied unless the gear ratio is changed.
Finish rolling of continuous length sections in a heated rolling line may be conducted in such manner that roll calibers are exchanged corresponding to variance of specific sizes of rolled products, or that a single roll caliber is used to depress the rolls into desired positions for providing rolled products with separate sizes (the technique is herein called "sizing-rolling").
The trouble is that, every time the parting of the mill rolls is adjusted at one or more roll stands in a rolling mill having a plurality of serially connected roll stands driven by a common drive, the material undergoes a sharp increase or decease in the tension between the stands. Consequently, product quality is adversely affected by an undesirable decrease in the diameters of products and breakage of rolled material by tension, increment of products diameters by compressive force and buckling of rolled material. Hence, an extent of sizing with adjustment of depressed positions of rolls is limited.
As one of the conventional measures to prevent the foregoing from occurring in a rolling mill having a plurality of serially connected three-roll stands driven by a common drive, it has been proposed (e.g. by a brochure published by Kock, Germany) to use a rolling-mill technique characterized in that all stands are driven until the moment when the material is caught in a roll stand disposed at the upstream end, and that all but said roll stand are disengaged from said common drive by means of one-way clutches at the above-mentioned moment so as to allow the material to be thrusted into the idling roll stands, provided that the area reduction to be finally attained at a roll stand disposed at the downstream end is small.
This rolling technique does not provide a stable rolling in sizing-rolling operation at a higher area reduction ratio, for example, of 20% by use of two 3-roll rolling mills as disclosed in Japanese Unexamined Patent Publication No. 43702/1988 since tension applied to rolled material between rolling mills varies largely corresponding to the specific sizing amounts and the rolled material is subjected to a higher compressive force due to force rolling. For carrying out a stable sizing-rolling, it is required to reduce a sizing range, lessen intervals between the rolling mills for eliminating influences on rolled material with compressive force applied thereto and enlarge diameters of rolled products.
Let it be supposed that two roll stands are connected to a common drive through a gear transmission assembly, that a gear ratio set in the gear transmission assembly is such that the material undergoes no tension between the stands when they are operated with an area reduction of 20% to be finally attained at the roll stand disposed at the downstream side, and that the actual parting of the mill rolls is such that an area reduction which can be finally attained at the roll stand disposed at the downstream side is much smaller than 20%. When the two roll stands are operated under this condition, the material undergoes unusually high tension between the rolling mills, thereby decreasing the diameter of the rolled material and possibly breaking the same.
Also, when two rolling mills are connected with each other through their respective drive systems and a one-way clutch is disposed at the drive system of one rolling mill placed downstream, so that when rolled material is just caught in the roll stand disposed at the downstream side, this roll stand is disengaged from the drive by means of the one-way clutch. Then the roll stand disposed at the upstream side will have the effect of thrusting the material into the idling roll stand disposed at the downstream side. The result is that, when the parting of the mill rolls is set in such a manner that a large area reduction is finally attained at the roll stand disposed at the downstream side, the material between the stands will undergo unusually high compressive stress and will be buckled or subjected to wave motion.
Accordingly, the known 3-roll rolling method has a problem that it is not capable of rolling the material of a larger area reduction ratio, i.e., in a wider sizing range.